1. Field of the Invention
This invention relates to a real image mode variable magnification finder for attachment which is constructed to be independent of a photographing optical system as in a still camera or a video camera.
2. Description of Related Art
Real image mode variable magnification finders have been designed so that an intermediate image is formed inside an image erecting system to reduce the total length of an objective unit. As an example, a design is known that a Porro prism is divided into two pieces. When this design is used, as shown in FIG. 1A, the entrance window of an objective optical system 2, in contrast with FIG. 1B, can be located at a position lower than the window of an eyepiece 4. Thus, a shift Sh of the objective optical system 2 from the optical axis can be diminished to keep parallax with a photographic lens 7 to a minimum. In particular, it is known that a prism such that light from the objective optical system 2 is reflected upward and then back minimizes interference with a film mask 5 of a camera and is most suitable for use in reducing the thickness of the camera. Also, in FIGS. 1A and 1B, reference numeral 1 represents a finder unit; 3, a Porro prism; 6, a film; and 8, a camera case.
As will be obvious from Japanese Patent Preliminary Publication Nos. Hei 7-84184 and Hei 9-68739, it is known that, in order to place a mechanism member for changing the size of a field frame, the field frame is placed above the side face of an objective optical system including a prism, after light is reflected three times by the prism, and thereby space for the mechanism member can be provided.
Further, as set forth in Japanese Patent Preliminary Publication No. Hei 5-53054, it is known that, in order to increase the optical path length of the back focus section of the objective optical system, it is only necessary to use a prism whose entrance surface is concave.
In addition, as disclosed in Japanese Patent Preliminary Publication No. Hei 1-257817, a technique is known that, in a real image mode finder using a Porro mirror in an image erecting optical system, an eyepiece is fixed to a frame to prevent dirt particles from penetrating into an intermediate image, providing an enclosed structure.
However, each of Hei 7-84184, Hei 5-53054, and Hei 9-68739 which are mentioned above has the problem that the adhesion of dirt particles to a field lens located on the pupil side of the intermediate image cannot be prevented because the eyepiece is movable for diopter adjustment.
Japanese Patent Preliminary Publication No. Hei 4-51108 is capable of increasing the back focal distance of the objective optical system, but has the problem that the total length of the objective optical system becomes large and thus the thickness of the camera cannot be decreased.